1. Field of the Invention
The present invention relates to current control circuits for use with subscriber line interfaces in communications systems, and more particularly to current control circuits capable of maintaining a constant DC current flowing in a load irrespective of the magnitude of the load.
2. Description of the Prior Art
FIG. 2 shows an example of a prior known current control circuit for use with subscriber line interfaces in a telecommunications network. In FIG. 2, the current control circuit 1 is constructed from a RING terminal 2 acting as a connection terminal, a TIP terminal 3, a power feed resistor 4, a power supply terminal 5, a current drive circuit 6, a reference voltage input terminal 7, a resistor 8, a capacitor 9, and an AC signal input terminal 10. The current drive circuit 6 essentially consists of a power feed transistor 11 of PNP conductivity type, operational amplifier 12, and feedback resistor 13. Note that reference numeral 14 designates an associative load resistor, which may correspond to the internal resistance of a telephone as connected to the current control circuit 1.
Here, the RING terminal 2 is connected through the power feed resistor 4 to the emitter of the power feed transistor 11 constituting the current drive circuit 6 and also to the feedback resistor 13. The collector of power feed transistor 11 is connected to the power supply terminal 5. The base of the power feed transistor is connected to an output terminal of the operational amplifier 12. The feedback resistor 13 is connected to an inverting input terminal of the operational amplifier 12 and is also connected to the AC signal input terminal 10 via a series combination of the resistor 8 and capacitor 9. Further, a non-inverting input terminal of the operational amplifier 12 is connected to the reference voltage input terminal 7. Furthermore, the load resistor 14 is connected between the RING terminal 2 and TIP terminal 3. The circuit connected to the TIP terminal 3 is not directly pertinent to the invention described herein, and therefore a detailed discussion of this circuit is omitted.
In the current control circuit 1 thus configured, a DC current is input from the TIP terminal 3 and flows into the power supply terminal 5 through the load resistor 14, RING terminal 2, power feed resistor 4, and power feed transistor 11. The operational amplifier 12 operates to drive the power feed transistor 11 in such a way as to force a voltage at the emitter of the power feed transistor 11 to be equal to a voltage input from the reference voltage input terminal 7 to the non-inverting input terminal of operational amplifier 12, thereby allowing such DC current to flow into the power feed transistor 11.
On the other hand, an AC signal such as an audio signal flows from the AC signal input terminal 10 into the TIP terminal 3 via the capacitor 9, resistor 8, feedback resistor 13, power feed resistor 4, RING terminal 2 and load resistor 14.
However, in the above example, the DC current flowing in the load resistor 14 varies with a change in resistance value of the load resistor 14. Especially, in the state in which the load resistor 14 is low in resistance value, DC current flowing in load resistor 14 increases causing an increase in the burden imposed on the power supply of current control circuit 1. A problem thus arises in that the current capacity of the power supply is required to be increased in advance in order to accommodate such a circumstance. Another problem is that the power supply's allowable electric power is also required to be increased in advance by taking account of the fact that large DC current might similarly flow in the power feed resistor 4 and power feed transistor 11, which would increase the cost of the current control circuit.